Ultra Wide Band (UWB) devices are designed to operate wirelessly at low signal levels over a wide frequency range, without interfering with other devices or services. Typically, the modulation scheme used by UWB devices is Orthogonal Frequency Division Modulation (OFDM), in which multiple subcarriers (sometimes referred to as tones) orthogonal to each other are used for modulating data.
The amount of transmission power allowed by UWB devices is usually restricted by government regulation. For example, in the United States, the FCC approved a limit of −41.3 dBm/MHz in the frequency band of 3.1-10.6 GHz. Thus, industry standards such as the WiMedia™ Specification follow the limitation and require devices to be designed to operate at or below the limit. FIG. 1A illustrates the power spectrum of a UWB device that has a flat transmission power spectrum. The center frequencies of the tones are shown for purposes of simplicity. In practice, the tones have a sin(f)/f profile.
In other countries or regions, however, different regulations may lead to different requirements. For example, in Europe and Japan, UWB devices may be restricted to a maximum transmit power of −70 dBm/MHz for certain frequency regions to avoid interfering with devices implementing other standards. FIG. 1B illustrates an ideal power spectrum of a UWB device that meets the lower transmission requirement in a specific frequency region.
To achieve a lower transmit power level in certain frequency ranges, UWB transmitters can be designed to turn off completely in these frequency ranges (or UWB frequency bands that contain the frequency ranges). This, however, will typically compromise the performance of the UWB devices since the devices will likely be forced to operate in alternative bands, thus limiting the total integrated transmit power, decreasing the number of simultaneously operating piconets, and reducing overall capacity.
Alternatively, the UWB transmitters can be designed to transmit at or below the specified lower power level in the designated frequency ranges, while maintaining their normal transmit power level for the rest of the frequency spectrum. For example, a WiMedia UWB transmitter can transmits at a power level of −41.3 dBm/MHz or less in most of the frequency spectrum, but at −70 dBm/MHz or less in frequency ranges shared by WIMAX or 4G devices. The power reduction can be achieved by generating a notch in the spectrum in specific frequency ranges. An ideal notch in the spectrum is shown in FIG. 1B.
In practice, generating a deep notch in the spectrum is a non-trivial matter because of the side lobes generated by OFDM tones in the frequency domain. Turning off tones with frequencies that fall within the notch nulls the effects of these tones. Tones centered at neighboring frequencies, however, contain power at other frequencies and contribute power within the notch. FIG. 1C illustrates the power contribution made by neighboring tones. As shown in this example, neighboring tones such as 102-106 have power profiles according to the functions sin(f)/f (shown in dashed lines). The side lobes of these tones fill the notch spectrum at various places, forming spurious signals (also referred to as residual tones) such as 108 and 110. The resulting power level inside the notch can exceed the required maximum due to the power contained in the residual tones.
In “Active Interference Cancellation Technique for MB-OFDM Cognitive Radio”, Yamaguchi introduced a method of generating a deep notch in the spectrum. According to this method, Active Interference Cancellation (AIC) tones are placed at the edges of the null region, and the amplitude and phase are calculated to cancel out the interference from other tones. In this way, nulls of depth −70 dBm/MHz or greater can be generated. The drawback of this method is that the resulting AIC tones may be of greater amplitude than the neighboring tones, therefore the entire band may have to be reduced to keep the large AIC tones below the −41.3 dBmi/MHz limit. This problem is illustrated in FIG. 1D, where the AIC tone pair 120 and 122 have greater amplitudes than data tones such as 124-130. In some cases, to remedy the large AIC tone, the average transmit power is reduced by 5-10 dB, and consequently the performance of the transmitter is also degraded. It would be desirable to have a way to cancel the interference without significantly reducing the performance of the transmitter.
Further, existing techniques for determining AICs are often computationally intensive, therefore costly to implement in hardware such as Application Specific Integrated Circuit (ASIC). It would also be useful to have an interference cancellation technique that is less computationally intensive and can be more easily implemented in hardware.